The present invention relates to the field of actuators, and in particular, direct drive actuators employing a radial magnetic field acting on a conducting coil.
There is currently a large effort devoted to the miniaturization of unmanned aerial vehicles (UAVs). Through rapid advancement in the miniaturization of essential elements such as inertial measurement units, sensors, and power supplies, Micro Air Vehicles (MAVs) have become a reality. However, little research has focused on the miniaturization of control surface actuators. Instead, MAV developers have used hobby-quality actuators. These actuators are typically too big, too heavy, too slow, inefficient and unreliable for use in MAVs. Therefore, there exists a need for reliable actuators that are designed to address the following issues: size, weight, bandwidth, torque, reliability, voltage, rate and position saturation.
The next generation of MAVs are described by the Defense Advanced Research Projects Agency (DARPA) as being less than 15 cm in length, width or height. This physical size renders this class of vehicle at least an order of magnitude smaller than any missionized UAV developed to date. Equally as important, the weight of the actuators should account for less than 5% of the total weight of the vehicle. Lincoln Lab investigated one example of a vehicle of this type. For a ten-gram concept vehicle, propulsion not only consumed 90 percent of the power, but also 70% of the weight budget. The remaining 30% of the weight budget accounted for the control surface actuators, as well as the flying structure, camera, atmospheric sensor array, and other avionics systems.
Past efforts to conform to MAV standards, such as Aerovironment""s Black Widow, have approached DARPA""s requirements with the flying wing approach. The flying wing achieves long flight duration; however, its low chord Reynolds number airfoils (30,000 to 70,000) operate in an aerodynamic regime far from the predictable aerodynamics of larger vehicles. The flying wing is highly susceptible to wind shear, gusts and roughness produced by precipitation. To achieve flight stability in this aerodynamic environment, the MAV must be capable of rapid actuation or have a high bandwidth. Intimately connected to the bandwidth, the torque requirement consists of maintaining an aerodynamic control surface in place. The actuators must not only be capable of rapid acceleration, but must also have adequate travel and peak angular velocity, thus satisfying the rate and position saturation requirements for MAV control surface actuation.
There are several approaches to determining the best actuator for MAVs. The current approach relies on available commercial off-the-shelf actuators. Given the current state of technology, many possible options, though substandard, exist to fulfill the microactuation requirements of MAVs. Among the possibilities are packaged servos, commercial motors, voice coil motors, HDD microactuators, and nanomuscles.
The first option is servo actuators. However, low bandwidth is the main drawback with packaged servo actuators. The approach in these actuators is to minimize the weight by using the smallest high-speed motors available, then gearing the speed down through an array of plastic gears while at the same time increasing the torque. In general, the equivalent motor inertia and frictional force on the driven shaft side increases by a factor of the gearing ratio squared, further reducing bandwidth. Such gearing not only introduces power loss, but also introduces backlash. Backlash causes unexpected dynamics in systems, such as the control surface for an aerial vehicle, which requires precise position control and undergoes frequent change in direction.
Further, the torque provided by commercial hobby servos is more than necessary for MAVs. Saturation occurs at relatively low speeds because the official specifications for these actuators do not indicate bandwidth; rather, the time for the actuator to travel 60 degrees is given. Such a degree of mismatch in performance requirements is unacceptable in a system with extremely tight size, weight and performance requirements.
Rather than using cased servos, using motors directly for actuation is another option. The advantage is that motors can be made very small. In particular, Faulhaber and Smoovy produce motors on the 2 and 3 mm scale. The overall disadvantage is that the motors are built for continuous operation and very high velocity at the expense of torque. This necessitates some form of transmission, and therefore, power losses and backlash between the motor and the final drive stage occur. Another drawback is that the very smallest motors are brushless polyphase devices, which require external controls.
Nanomuscles are linear actuators commercially manufactured near the size factor required for MAV applications. Nanomuscles are attractive devices for microactuation because they are small, light, and are capable of very large forces over adequate stroke (4 mm). The major drawback, however, is that the actuation time is about one-half of a second. Another drawback is that the nanomuscles are only capable of contraction, thus requiring two units for full actuation.
Among the many types of actuators such as speakers, rotary, etc., the voice coil actuator family also encompasses hard disk drive (HDD) actuators. The boom of the computer industry pushes for continual improvements in HDD actuators. The goal of the HDD manufacturers is higher data storage capacity achieved through increased head position resolution and bandwidth. The most common method for high bandwidth HDD actuation is the combination of a high travel, low-resolution voice coil actuator in series with a low travel, high-resolution microactuator.
The voice coil alone achieves high bandwidth through direct drive actuation and low arm inertia. The force of actuation in voice coil motors, as in all direct drive motors, is purely electromagnetic; the only source of friction is the support bearing for the arm or object being moved. The main drawback to the voice coil design is the heavy weight of non-moving components. For data storage, overall weight reduction is not a vital requirement; therefore, only portions of the magnetic field and current are used at any given time for actuation.
Among the most common microactuators are those used on the tips of read heads for HDDs. These microactuators are divided into two families: piezo and electrostatic. Advantages of these actuators include a high bandwidth on the order of kilohertz and a very lightweight and small package. On the other hand, the actuator is so small that the effective stroke only extends on the order of micrometers. Another drawback to HDD microactuators for MAVs is that both piezo and electrostatic slider actuators require near 80 Volts for full travel. Piezoelectric multilayer bender actuators provide higher travel on the order of a millimeter; however, they still require high voltages.
The present invention provides a high intensity radial field (HIRF) magnetic array and actuator employing direct drive technology, which operates particularly well in micro scale applications.
A nested magnetic array consistent with the invention comprises an outer magnet with a magnetization pointing in an axial direction; a middle magnet with a radial magnetization which is pointed either concentrically inward or outward and is perpendicular to the magnetization of the outer magnet; and an inner magnet with a magnetization pointed anti-parallel to the magnetization of the outer magnet.
In one embodiment, a permanent magnet actuator comprises a first magnetic array comprising nested outer, middle and inner cylindrical magnets, wherein the outer annular magnet of the first magnetic array has a magnetization pointing in an axial direction, the middle annular magnet of the first magnetic array has a radial magnetization which is pointed either concentrically inward or outward and is perpendicular to the magnetization of the outer annular magnet, and the inner cylindrical magnet of the first magnetic array has a magnetization pointed anti-parallel to the magnetization of the outer annular magnet; and a conductive coil having a current located within the volume of conductor, wherein the magnetic field of the first magnetic array is substantially radial and perpendicular to the current located in the conductive coil. The conductive coil may be located above or below the first magnetic array, depending upon the magnetization direction of the magnets in the magnetic array.
In another embodiment, a permanent magnet actuator further comprises a second magnetic array comprising nested outer, middle, and inner cylindrical magnets, the second magnetic array being located on the opposite side of the conductive coil from the first magnetic array, wherein the outer annular magnet of the second magnetic array has a magnetization pointing in an axial direction parallel to the direction of the magnetization of the inner cylindrical magnet of the first magnetic array, the middle annular magnet of the second magnetic array has a magnetization in the same direction as the middle magnet of the first magnetic array, and the inner cylindrical magnet of the second magnetic array has a magnetization anti-parallel to the magnetization of the outer annular magnet of the second magnetic array; wherein the conductive coil is disposed between the first and the second magnetic arrays, and wherein the magnetic field of the first and the second magnetic arrays is perpendicular to the current located in the conductive coil. The coil may comprise at least one wire having a plurality of turns.
In method form, a method for creating a magnetic force comprises creating a magnetic field engulfing a conductive coil, the magnetic field comprising the superposition of a first magnetic field curling from an inner ring of a magnetic array to an outer ring of the magnetic array, and a second magnetic field pointing radially outward from a middle ring of the magnetic array; and applying a current through the conductive coil.
The conductive coil may have a winding that is variously configured, e.g., pancake-shaped, solenoidal, or toroidal. The coil may comprise more than one winding (e.g., two windings wound in opposing directions) for use, e.g., in a two degree-of-freedom actuator, with independently controlled orthogonal axes.
Further, in an exemplary actuator consistent with the present invention, the arrays may be canted to permit the toroidal winding to expand, affording control over the spread of the magnetic field in the gap.